1. Field of the Invention
This invention relates to catalyst compositions and methods of producing auto exhaust catalysts that substantially reduce H.sub.2 S odor from the tail pipe, while still meeting catalytic performance requirements for three-way conversions.
2. Description of the Previously Published Art
With the advent of advanced catalysts for controlling auto exhaust emissions, typically containing a higher than usual level of rare earth oxides, especially ceria, the level of H.sub.2 S emission from tail pipes has significantly increased, to the extent that the strong H.sub.2 S odor can no longer be tolerated.
In practice, however, it is rather difficult to develop an auto exhaust catalyst that meets the requirements for both the long-term catalytic performance for three-way conversions as well as H.sub.2 S because of the following fact. The performance of the catalyst which has already been optimized for three-way conversions tends to be adversely affected by any chemical modification introduced to the existing catalyst to meet the requirement for H.sub.2 S.
U.S. Pat. No. 4,780,447 teaches that NiO and/or Fe.sub.2 O.sub.3 is useful in the control of H.sub.2 S in certain types of automobile catalytic converters. The catalyst in that patent is specific to pelleted-type auto catalysts. The invention to be described here is applicable to the more widely used washcoats supported on ceramic or metal monoliths and has limited applicability to pelleted-type catalysts. Also the NiO and Fe.sub.2 O.sub.3 components described in U.S. Pat. No. 4,780,447 are added by an impregnation process. As will be described the additives of this invention are added as a bulk oxide to the catalyst composition.
European patent application No. 244,127 published Nov. 11, 1987, describes a three-way catalyst (TWC) comprising a Pt-group metal and sufficient Ni or NiO to reduce H.sub.2 S when the catalyst is used under fuel-rich conditions.
U.S. Pat. No. 4,738,947, discloses a TWC comprising alumina, a Pt-group metal, and a rare earth metal oxide plus a "p-type" metal oxide (meaning oxide of Co, Ni, Fe, Mn, Cr, etc.). In this patent the "p-type" metal oxide is deposited by an impregnation process on cerium oxide powder present in the catalyst. This combination is claimed to have improved catalytic performance, but no indication of reduced H.sub.2 S emissions is given.
U.S. Pat. No. 4,591,580 discloses a stabilized and doubly promoted platinum group catalyst that could be used in conjunction with an oxide additive. The alpha-alumina formation was greatly reduced by stabilizing a transitional alumina support with lanthana or La-rich rare earth oxides while still maintaining high catalytic performance. This catalyst comprises four material components which are (1) support or carrier, (2) stabilizer, (3) promoters which are preferably CeO.sub.2 and alkali metal oxides and (4) catalytic metals. The description of this catalyst did not have any discussion of the H.sub.2 S problem in auto emission control. There was disclosed an optional embodiment in which Ni could be added as 0.5 to 20 wt. % NiO.
U.S. Pat. No. 4,760,044 teaches a method for reduced H.sub.2 S emissions based on the use of compounds which form stable sulfides under fuel rich conditions. Compounds that form such stable sulfides include Ni, Co, Zn, and Cu. Such elements are supported on oxides such as ceria, titania, or zirconia in this patent and introduced on the said support by an impregnation process.
As indicated by the prior art, Ni or NiO is a superior H.sub.2 S gettering agent. However, in this use nickel may be gradually lost to the atmosphere, and the amount of loss, though small, may cause problems vis-a-vis anti-pollution regulations in some areas or cause potential health concerns. (OSHA has stated nickel to be carcinogenic). The instant invention aims at avoiding nickel problems without sacrifice of catalyst performance. We accomplish this by the use of a metal ferrite as an H.sub.2 S gettering agent. Ferrites are binary oxide compounds composed of iron oxide and the oxide of one other transition metal (e.g. Cu, Co, Mg, Ni). The use of nickel ferrite (NiFe.sub.2 O.sub.4) as herein disclosed shows performance similar to that of NiO but at a substantially reduced level of Ni, since Ni is only about one-third of the total weight of NiFe.sub.2 O.sub.4. Other non-Ni containing ferrites such as CoFe.sub.2 O.sub.4 and CuFe.sub.2 O.sub.4 as herein disclosed show performance similar to that of NiO. The effectiveness of the ferrites is rather surprising, since the aluminate analogs (CoAl.sub.2 O.sub.4, NiAl.sub.2 O.sub.4) are substantially without effect in H.sub.2 S gettering. Also the ferrite additives have less impact on catalytic performance than the simple transition metal oxides such as CoO or CuO.
The use of ferrites such as ZnFe.sub.2 O.sub.4 and CuFe.sub.2 O.sub.4 in scavenging H.sub.2 S from gas steams derived from coal combustion in the power generation industry is known. See, for example, Tamhankar et al. in Ind. Eng. Chem. Process Des. Dev., 25, 429 (1986). Such ferrite function differs from ours in important respects. For example, in this power generation application the ferrite-H.sub.2 S adduct must be regenerated in a separate operation using an oxygen-containing stream, a process convenience which is impossible in an auto catalyst, where the ferrite must be regenerated in situ by the very effluent that "poisons" it.
3. Objects of the Invention
It is an object of this invention to chemically modify the auto exhaust emission control catalyst formulation to virtually eliminate the H.sub.2 S odor while still maintaining adequate catalyst performance for controlling hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxides (NO.sub.x).
It is a further object of this invention to disclose a method whereby a slight and yet effective chemical modification can make a TWC highly efficient in lowering H.sub.2 S emission from the tail pipe while retaining virtually all or adequate catalyst efficiency for three-way conversions.
It is a further object of this invention to add an H.sub.2 S gettering ingredient to an auto exhaust emission control catalyst while maintaining the activity for the control of the other emissions.
It is a further object of this invention to add an H.sub.2 S gettering ingredient to an auto exhaust emission control catalyst in a manner so as to achieve a maximum effect on H.sub.2 S control using a minimum level of H.sub.2 S gettering ingredients.
A still further object is to add an H.sub.2 S gettering ingredient to an auto exhaust emission control catalyst, which ingredient is either nickel-free or contains only a minimal amount of nickel.
Another objective is to provide an efficient TWC meeting regulatory Ni maxima.
These and further objects will become apparent as the description of the invention proceeds.